“The Mass in B Minor is a musical setting of the Latin Mass. Although parts of the Mass in B Minor date to 1724, the whole was assembled in its present form in 1749, just before the composer’s death in 1750.
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Due to its length–over two hours of music–it was never performed as part of a church liturgy. The first performance seems to have been after Bach’s death, when his son Carl Philipp Emanuel Bach performed the Gloria section (but not the entire Mass) in Berlin. Large-scale performances of the entire Mass in B Minor were not staged until the 19th century.”

Latin Text:

Agnus dei
qui tollis peccata mundi
miserere nobis.

Translation:

Lamb of God,
who takes away the sins of the world,
have mercy upon us.